Electroluminescent array and method and apparatus for controlling discrete points on the array

ABSTRACT

An electroluminescent array and a method and circuit arrangements for energizing discrete elemental points on the electroluminescent array. The electroluminescent array may include a predetermined number of row circuit lines and a predetermined number of column circuit lines arranged in a crossgrid X-Y pattern to form a circuit juncture at each crossing of the column and row circuit lines, and a threshold operated electroluminescent circuit is connected at each of the junctures. Each threshold operated electroluminescent circuit most advantageously comprises a bidirectional threshold switch having a time delay characteristic, an electroluminescent element, and a resistor which may be a discrete element or formed by the circuit resistance. A power supply and selector control circuit means are connected to the electroluminescent array such that operating potential is continuously applied to the array but no electroluminescent circuit is energized until a start pulse of predetermined configurations is applied to a selected row circuit line and/or to a selected column circuit line and only then is the electroluminescent circuit at the juncture of the selected row and selected column circuit lines energized and remains energized while the other electroluminescent circuits of the array, if not selected, remain deenergized. The method and circuit arrangements together with the electroluminescent circuits used in the array provide means for applying uniform voltage to each and every electroluminescent element therein regardless of differences of threshold voltage value or inherent time delay of the threshold switch devices thus providing uniform lumination of the discrete electroluminescent elements. An energized electroluminescent circuit continues to be energized until a stop pulse is applied to selected row and column circuit lines and then only the electroluminescent circuit at the juncture thereof is de-energized, and other previously energized electroluminescent circuits remain in the energized state.

United States Patent 1 Fleming [54] ELECTROLUMINESCENT ARRAY AND METHOD AND APPARATUS FOR CONTROLLING DISCRETE POINTS ON THE ARRAY [75] Inventor: Gordon R. Fleming, Pontiac, Mich.

[73] Assignee: Energy Conversion Devices, lnc.,

- Troy, Mich.

[22] Filed: May 16, 1969 [21] Appl. No.: 825,153

[52] US. Cl. ..315/l69 TV, 313/108 B, 315/169 R [51] Int. Cl. ..H05b 33/00, G09f 13/22 [58] Field of Search ..3l3/l08 B;

315/166, 169, 169 TV; 340/166, 166 EL [56] References Cited UNITED STATES PATENTS [5 7] ABSTRACT An electroluminescent array and a method and circuit arrangements for energizing discrete elemental points on the electroluminescent array. The electroluminescent array may include a predetermined number of row circuit lines and a predetermined number of column circuit lines arranged in a cross-grid X-Y pattern to form a circuit juncture at each crossing of the column and row circuit lines, and a threshold operated electroluminescent circuit is connected at each of the junctures. Each threshold operated electroluminescent circuit most advantageously comprises a bidirectional threshold switch having a time delay characteristic, an electroluminescent element, and a resistor which may be a discrete element or formed by the circuit resistance. A power supply and selector control circuit means are connected to the electroluminescent array such that operating potential is continuously applied to the array but no electroluminescent circuit is energized until a start pulse of predetermined configurations is applied to a selected row circuit line and/or to a selected column circuit line and only then is the electroluminescent circuit at the juncture of the selected ro w an d sel ected column circuit lines energized and remains energized while the other electrolu- .minescent circuits of the array, if not selected, remain deenergized. The method and circuit arrangements together with the electroluminescent circuits used in the array provide means for applying uniform voltage to each and every electroluminescent element therein regardless of differences of threshold voltage value or inherent .time delay of the threshold switch devices thus providing uniform lumination'of the discrete electroluminescent elements. An energized electroluminescent circuit continues to be energized until a stop pulse is applied to selected row and column circuit lines and then only the electroluminescent circuit at the juncture thereof is de-energized, and other previously energized electroluminescent circuits remain in the energized state.

24 Claims, 18 DrawingFigures INPUT "hs shgn [mama COLUMN 40 5a Ecr 5W.

PATENTEU JAN 2 I973 SHEET 3 BF 9 IDEAL/ZED 5EQUENCE 0F OPERATYOA/S m mm r M M M W W MT aw w w m WP 4% MN 5 w 3 emw m M w rwm T fin M? v 5 if w 3/ T M Mfl a v E m f I R .7 7 LI LII. m 3 LI /0 7 6 I 5 7 La 1 c IFJI I a w O t O TIME PATENTED JAN 2 I975 SHEET 9 [IF 9 MM flame Mg ELECTROLUMINESCENT ARRAY AND METHOD AND APPARATUS FOR CONTROLLING DISCRETE POINTS ON THE ARRAY This invention relates generally to panel type information display arrays, and more particularly to two dimensional display devices for visual reproduction of electronic signals, and to the method and apparatus for controlling such arrays. Specifically, this invention is directed to energizing and de-energizing discrete selected elemental points on an electroluminescent array wherein each discrete electroluminescent point of the array is controlled between ON and OFF states without influencing other non-selected discrete electroluminescent points on the array.

l-leretofore, many problems have been encountered in the development of electroluminescent display matricies of the two dimensional display type. Some of the problems encountered are the result of certain minimum requirements which must be met to provide reliable energization and de-energization of discrete electroluminescent points. Some of the requirements are: the electroluminescent element must be controlled in accordance with stored video signal information; and the switching device used to control the electroluminescent element must exhibit a well-defined threshold voltage value, i.e. the tolerance of the threshold voltage value of a multitude of threshold switch devices must be very close. In addition, the electroluminescent elements must exhibit low power consummation, and must be amenable to batch-fabrication in geometries that can be assembled into display panels.

A common type of construction for an electroluminescent display panel is the cross-grid of X-Y type. The parallel plate electrodes of this type of construction are divided in strips and oriented at right angles to each other. Although such panels generally display a cartesian coordinated system, they may, if desired, display a polar coordinate system or any other system desired.

In this type of display panel an alternating current voltage is applied between the X and Y cross-grids to energize the electroluminescent elements at the juncture of the cross-grids. In practice, X-Y display panels of this type possess many such junctures each of which constitutes a connection means for a picture element to form a point on the array. Energizing selected picture elements would make possible the display of television pictures, or other information patterns or numbers. However, many problems have been incountered in trying to achieve a faithfully produced or reproduced image on display screens of the X-Y display'panel type. One such problem is the requirement of sequentially applying operating potentials to successive rows or columns, this being necessary to prevent energization of undesired electroluminescent elements.

Unfortunately, sequential scanning of the cross-grid electrode on an X-Y display with operating potentials is impractical. One disadvantage of this approach is that the brightness of each electroluminescent element is substantially reduced because the brightness of each electroluminescent element depends on the number of times that element is energized by the operating potential during the sequential operation. For example, if five elements are scanned and each element is excited for only one-fifth of the scanning period than that element emits only one-fifth of its normal light output. If a 20 element display is energized then each element is energized one-twentieth of the time of the scanning period and the light output of the element is reduced even further. Therefore, as the number of elements increase the brightness of each element is effectively reduced. Furthermore, when cross-grid electrodes are energized by sequential scanning the operating voltage and frequency must be high to provide a sufiiciently high brightness from the electroluminescent elements.

Another disadvantage of X-Y matrix display panels of the prior art is that the spurious cross-image or ghost effect. This is the result of energizing a pair of orthogonal electrodes with a sufficient operating voltage to gain access to the electroluminescent element at the juncture of the electrodes but which also causes other elements along each of the orthogonal electrodes to be partially energized as a result of the sustained but reduced voltage on each line thus producing the low visible glow. Nonlinear resistors have been used with this type of display panel to reduce the visible glow caused by the cross-image effect and to improve the contrast ratio of the image.

In addition to the cross-image problem of prior art display panels there exists a scanning and storage problem and many different approaches have been taken to solve these problems, as for example, using image amplifiers, ferro-electric control circuits, nonlinear resistor gate controls, and switching matrices using transistors. However, each of these methods require either large bulky circuitry to operate the display panel or thedisplay panel operates in an adverse manner.

In another type of electroluminescent display panel each electroluminescent element is individually controlled by separate control leads connected to each element. However, in display panels having a large number of discrete electroluminescent elements the number of control leads required to operate the panel becomes unreasonable for practical circuitry construction. That is, the switching requirement for a display panel of this type is very complex and the display panel becomes bulky. In addition, there is the problem of making electrical connections with each of the electroluminescent elements while maintaining a moistureproof seal about the element.

In electroluminescent display panels which use discrete electroluminescent elements in combination with discrete current controlling devices such as, for example, four layer diodes, thyristors or the like, certain other problems are encountered. For example, some of the current controlling devices which are theoretically possible to use in combination with electroluminescent displays cannot be bulk fabricated in thin films or layers thus requiring the display panel and its associated circuitry to be bulky and costly. Additionally, the requirement to manufacture a multitude of current controlling devices with substantially the same electrical properties is very expensive in that this requires testing and selecting from a very large number of devices only those devices which meet very closely the electrical characteristics required for proper operation of the X-Y array. Therefore, in the case where threshold switching devices are used to control current through the electroluminescent elements each of the threshold switching devices are required to operate substantially at the same threshold voltage value, i.e. very close tolerances are required. This is necessary for two very important reasons. First, a relatively large variation of threshold voltage values will result in erratic operation of the electroluminescent array in that a threshold voltage value below the required tolerance willresult in unselected electroluminescent elements being energized and a threshold voltage value above the required tolerance will result in selected electroluminescent elements remaining de-energized after a selecting voltage has been applied thereto. Another problem of using threshold switching devices with dissimilar threshold voltage values, particularly when used in combination with electroluminescent arrays for energization by alternating current sine waves, is that the various electroluminescent elements will be energized at different amplitude values of the applied sine wave voltage thus applying different voltage to the various electroluminescent elements which causes corresponding different light intensities from these elements. This undesirable condition will result in a nonuniform light emitting display pattern from the electroluminescent array.

It is believed that many of the above undesirable results and related problems are due to a great extent as a result of designing such electroluminescent array on the basis of an ideal theoretical switch device, which, up to now, is non-existent. By assuming the ideal theoretical threshold switch device for use in electroluminescent arrays many circuit parameters are selected or designed which, when combined with the actually available non-perfect switches, cause the electroluminescent array to operate in an erratic and undesirable manner.

To overcome many of the above disadvantages, this invention uses threshold voltage switching devices connected in series with an electroluminescent element to form the many discrete points on an electroluminescent array and provides control means for continuously applying power to the electroluminescent array and selectively applying start and stop pulses to the array to energize and de-energize one or more desired points thereof, and which control means is designed to be compatable for use with actual rather than theoretical switch devices.

The switching devices used in this invention are one layer type threshold semiconductor devices each having substantially identical conduction characteristics for positive and negative applied voltages but which may have slight different threshold voltage values with respect to one another. The devices initially present a very high resistance in response to an applied voltage of either polarity below an upper threshold level and a very low resistance in response to an applied voltage of either polarity at or above an upper threshold level, the change from the high to the low resistance condition occurring after an inherent time delay of the switching devices but once switching begins it is substantially instantaneous. The threshold semiconductor devices automatically reset themselves to their high resistance state when the current therethrough drops below a minimum holding current value of each particular device which is near zero but the switching devices has a substantially reduced threshold voltage value immediately after it is rendered non-conductive, and after a relatively short recovery time delay during which the threshold voltage value progressively increases until 5 the normal threshold voltage value is again reached. Semiconductor materials used to form such threshold switching devices most advantageously are of the type disclosed in U. S. Pat. No. 3,271,591 and sometimes referred to therein as mechanism devices without memory. By varying the semiconductor composition or the treatment of the material disclosed in the above mentioned patent, the upper and lower threshold levels and the blocking or leakage resistance thereof are readily varied to obtain the desired range of conditions necessary for proper operation of the electroluminescent arrays constructed in accordance with this invention. Blocking resistance values of the order of one to megohrns and higher are readily obtainable, as well as somewhat lower blocking resistance values.

it has been discovered that the above type of threshold switching devices have inherent characteristics which are at variance with the threshold ideal switch, and these characteristics have been carefully studied. An inherent characteristic of particular importance is that of a varying time delay between the time a threshold voltage is applied to the switch and the time the switch actually changes from its high resistance blocking condition to its low resistance conducting condition, but then the switching occurs it is substantially instantaneous, as for example in the order of nanoseconds. However, the turn-on time delay of these devices will vary with changes in applied voltage in excess of the threshold voltage value of the particular devices involved, an increase in applied voltage from the threshold voltage value to a greater value causing a decrease in the turn-on time delay. Therefore, if a voltage pulse having an amplitude equal to or greater than the threshold voltage value of the switching device involved is applied thereto but exists for a period of time less than the inherent time delay corresponding to that particular voltage amplitude, the threshold switching device will not be rendered conductive. Hence, if an operating potential of alternating current voltage is applied the threshold switching devices used in this invention and which alternating current voltage is of a sufficiently high frequency so that periodic pulses of the applied frequency have a time duration less than the inherent time delay corresponding to the amplitude of the applied voltage, then a voltage value in excess of the threshold voltage value of these switching devices may be applied thereto without rendering these switching devices conductive. Therefore, when a multitude of such threshold switching devices are incorporated in the formation of an electroluminescent display array the threshold voltage values of these devices need not be the same, and in fact some or all of the devices can have a threshold voltage value less than the amplitude of the applied voltage without causing erratic operation of the display array, which is a surprising and unexpected result heretofore unobtainable with the arrangements of the prior art.

Yet another inherent characteristic of the threshold switching devices most advantageously used in this invention is that of a time delay in the recovery of the threshold voltage value back to the original or normal threshold voltage value after the switching devices are turned off as a result of decreasing current through the switching devices below the minimum holding current. That is, there will exist a substantially diminished threshold voltage value after the switching devices are turned off and which will increase with increasing time back to their original or normal threshold voltage value. Therefore, if a pulse of voltage is applied to any one the the threshold switching devices within the recovery time delay period after it is rendered non-conductive, this pulse need only have an amplitude equal to the then existing threshold voltage value to again render the switching device conductive. If the operating potential applied to the switching devices is an alternating current voltage of sufficient frequency so that each successive periodic pulse of the applied potential will reoccur within the inherent time delay for full recovery of the switching devices, these switching devices -will be continually rendered conductive on each half cycle of the applied voltage even if the applied voltage is substantially below the threshold voltage value of the switching devices involved.

By utilizing the inherent tum-on time delay and recovery time delay of the threshold switching devices disclosed in said patent to Stanford R. Ovshinsky many novel and unobvious advantages are realized. The need for selecting a multitude of threshold switching devices with substantially the same threshold voltage value is eliminated in that the efficient and reliable operating range of these switching devices (the tolerance range) is selectively increased in proportion to increasing frequency of the applied operating potential. Also these switching devices are readily batch fabricated as thin films or layers in contact with flat deposited surfaces of electroluminescent material thus making it possible to form flat relatively thin display screens.

Also, by utilizing the above mentioned threshold switching device in accordance with this invention the problem of inconsistents light intensities from various electroluminescent elements is eliminated. This is made possible by operating the electroluminescent elements and the threshold switching devices at a frequency and amplitude such that only the maximum portions of the successive alternating current voltages applied thereto will render the switching devices conductive which will apply substantially the same voltage to each and every one of the electroluminescent elements. This can be accomplished by using either sine wave or square wave voltages, or any other wave shape having a relatively fast rise time and a relatively long time duration of maximum potentials. Since electroluminescent materials will produce light proportional to the voltage applied thereto this aspect of the invention will provide the same light output of the multitude of electroluminescent elements to faithfully produce or reproduce images on the screen formed by the electroluminescent array of this invention.

Briefly, the electroluminescent array and method and apparatus used in this invention provide means for addressing selective elemental points on a display panel and wherein the electroluminescent element at that point is maintained energized after the address signal is removed. Each electroluminescent point on the display panel is preferably formed by a circuit which includes an electroluminescent element, a resistor which may be a discrete element or formed by circuit resistance, and a voltage sensitive symmetrical threshold switch which has inherent turn-on time delay and inherent recovery time delay characteristics.

One aspect of this invention is the ability to control a plurality of electroluminescent elements with a substantially uniform applied voltage across each of the multitude of electroluminescent elements, and providing uniform brightness, regardless of the wide variations of tolerance of threshold voltage values of the switching devices used. This is accomplished by making use of the inherent time delay characteristics of the threshold switching devices disclosed in the above said patent. All of the threshold switching devices in the array, whether in the X-Y cross grid arrangement or in any other arrangement, are driven by a supply voltage of a particular preselected frequency such that the threshold switching devices will be rendered conductive only during slight voltage variation near the maximum value portions of the applied alternating current voltage, the frequency being selected so that the maximum crest of each half cycle will have a time duration sufficiently long so as to be equal to or greater than the inherent time delay of the threshold switching device having the longest time delay of any particular group of switching devices used.

With respect to the electroluminescent element, the specific electrical parameter utilized in the circuit operation is that of the capacitive characteristic. That is, the electroluminescent element is operated as if it were a capacitor. The resistor is used to limit the current flow through the circuit and to provide an RC time element constant for charging current flow through the quasi capacitor. The voltage sensitive threshold switch is most advantageously of the type disclosed in US. Pat. No. 3,271,591 issued to Stanford R. Ovshinsky on Sept. 6, 1966, and is sometimes referred to therein as a Mechanism Device without memory. One of the great advantages of using the switching device disclosed in the above reference patent is the fact that the type of switching materials disclosed in said patent lend themselves to bulk manufacture of switches which may be deposited as films or layers on the back of flat relatively thin electroluminescent arrays. Therefore, integral construction of electroluminescent arrays and their associated switching circuitry is made possible. Another great advantage is that the need for testing and selecting a multitude of current controlling devices so that the electrical characteristics of the devices used will be within a relatively narrow range of tolerances is eliminated.

The invention uses to advantage the fact that a resonant circuit may be constructed by using a capacitor and a threshold switching device, and the resonant circuit so constructed has two operational stable conditions. That is, while continuously maintaining a constant voltage of predetermined amplitude on all of the electroluminescent elements of the array the individual elements or lamps of the array may be either ON or OFF when operated by the method of this invention.

uses the cross-grid construction wherein a plurality of electrodes are arranged in X-Y relation to provide non-connected junctures at the crossing of each electrode and whereat electroluminescent circuits are connected at the junctures and are selectively energized. Alternating current voltage of a predetermined frequency is continuously applied to the X group of electrodes and to the Y group of electrodes without causing energization of the electroluminescent circuits until a particular circuit is selectively energized and thereafter that circuit remains energized by the continuously applied alternating current voltage.

In an alternate arrangement of this invention the alternating current voltage generated is such that the varying voltage applied to the X group of electrodes is 180 degrees out of phase with respect to that applied to the Y group of electrodes so that the potentials applied to the X and Y groups of electrodes are of opposite polarity. Therefore, at points in time during the peak value of each half cycle of the alternating current voltage the voltage drop across each junctures of the grid construction is twice that of the peak value of the voltage applied to a given electrode. The normal threshold value of the multitude of threshold switching devices is preferably selected to be greater than about twice the peak value of the alternating current voltage. Therefore, none of the threshold switching devices are rendered conductive by the continuous application of alternating current'voltage regardless of the frequency thereof. However, when a single start pulses having an amplitude greater than the threshold voltage value of switching devices, and a time duration equal to or greater than the inherent time delay for that particular voltage value, are applied to selected ones of the X and Y lines, the electroluminescent circuits at the junctions of these lines are rendered operative and will be excited or energized at the frequency of the continuously applied alternating current voltages. The start pulse may be generated to coincide with the peak value of the altemating current voltage or to coincide with the zero crossing of the alternating current voltage so long as it is of sufficient amplitude and time duration to exceed the threshold voltage value of the threshold switching devices used. Once the switching devices are rendered conductive, the electroluminescent elements, acting like capacitors, are charged by the start pulse or pulses and hereafter the applied alternating current voltage together with the charge on the electroluminescent elements are combined and the total voltage applied to each of the previously conductive threshold switching devices is sufficient to render the switching devices conductive during each half cycle of the applied A.C.

' voltage. This action' will continue until a stop signal is applied to the X and Y lines to discharge the electroluminescent element or otherwise nullify the applied voltage so that the next preceding pulse of alternating current voltage will not render the switching device conductive, the time duration of the stop signal or nullifying signal, whichever the case may be, being sufficiently long to ensure'that the threshold switching device will recover to a threshold voltage value which is discharge the electroluminescent element so that the switching device will no longer be periodically energized by the continuously applied alternating current voltage. The start and stop address signals need by applied only once to the selected electrodes and thereafter the electroluminescent circuit at the juncture of the selected electrodes will remain in one of its two stable operational conditions.

Many objects, features, and advantages of this invention will be more fully realized and understood from the following detailed description when taken in conjunction with the accompanying drawings wherein like reference numerals throughout the various views of the drawings are intended to designate similar elements or components.

FIG. 1 is a schematic diagram showing a simplified circuit for manual selection of discrete points on an electroluminescent array constructed in accordance with the principles of this invention;

FIG. 2 is a simplified block diagram illustrating automatic operation of an electroluminescent array in accordance with this invention; I

FIG. 3 is a schematic diagram illustrating the component parts of an electroluminescent circuit as used in accordance with this invention;

FIG. 3A is a graphic representation of the turn-on time delay characteristic of the switching devices used in this invention;

FIG. 3B is a graphic representation of the recovery time delay characteristic of the switching device used in this invention;

FIG. 3C illustrates an alternating current voltage which renders the threshold switching devices conductive at the maximum voltage portions of the applied voltage in accordance with this invention;

FIG. 4 is a series of waveforms illustrating one mode of operation of the automatic control circuit means of FIG. 2;

FIG. 5 is a simplified block diagram illustrating one kind of circuit arrangement for controlling electroluminescent arrays in accordance with this invention;

FIG. 6 is a partial schematic and partial block diagram illustrating the power drive circuit of FIG. 5 for developing first and second alternating current voltage one of which is degrees out of phase with respect to the other;

FIG. 7 is a series of waveforms at various circuit points of FIG. 6;

FIG. 8 is a schematic diagram illustrating one means for developing start and stop pulse signal information in accordance with the principles of this invention;

FIG. 9 is a series of waveforms illustrating the alternating current voltage applied to the electrolu minescent array, when using the circuit arrangement of FIGS. 5, 6 and 8;

FIG. 10 is a schematic diagram showing another simplified circuit for normal operation of an alternate form of an electroluminescent array operated by the principles of this invention;

FIG. 11 is a block diagram illustrating another form of this invention;

FIG. 12 is a block diagram of an alternate form of the apparatus shown in FIG. 11;

FIG. 13 is a partial block diagram and a partial schematic diagram of the power supply used to operate the circuit arrangements shown in FIGS. 11 and 12;

FIG. 14 illustrates a series of the waveforms used to operate the electroluminescent array of FIGS. 11 and 12 in accordance with another method of this invention; and

FIG. is a perspective view showing one form of construction of an electroluminescent array according to this invention.

Now referring to FIG. 1 there is illustrated a circuit arrangement which provides for manual selection of discrete electroluminescent points of an electroluminescent array for operation in accordance with the principles of this invention. An electroluminescent array 10 includes a plurality of electrodes or lines 11, 12, 13 and 14 extending in the X direction of an X-Y coordinate system and a plurality of lines 16, 17, 1% and 19 extending in the Y direction to form a plurality of circuit junctures across which there are connected electroluminescent circuits 20, it being understood that there may be as many X and Y lines as desired. A particular electroluminescent circuit 20 will be identified by defining the circuit lines forming the juncture at which the electroluminescent circuit is connected. Therefore, the electroluminescent circuit 20 at juncture 11-16 is that circuit located at the top left hand comer of the array 10, as seen in FIG. 1.

An alternating current voltage from a source 21 is continuously applied to the X and Y lines of the electroluminescent array 10 and the amplitude of the applied alternating current voltage is maintained below the initial starting voltage required to energize any of the electroluminescent circuits 20, and the frequency of the applied voltage is selected such that the maximum crest portions of each half cycle will persist for a period of time equal to or greater than the inherent time delay of the threshold switching devices after they are initially rendered conductive. A voltage divider network, comprising resistors 22, 23 and 24, has one end thereof connected to a line 26 and the other end thereof connected to a line 27 which are the power feed lines for the X electrodes 11-14 and the Y electrodes 16-19 respectively. The resistors 22, 23 and 24 may be replaced with a double ended auto-transformer having the center portion thereof connected to the power source 21. Connected between the line 26 and each of the electrodes 11, 12, 13 and 14 are resistors 28, 29, 30 and 31 each of which has an ohmic value sufficiently high to provide suitable isolation between adjacent ones of the electrodes 11, 12, 13 and 14. Similarly, connected between the line 27 and each of the lines l6, l7, l8 and 19 are resistors 32, 33, 34 and 35 respectively, and the resistors 32-35 also provide suitable isolation between the adjacent ones of the electrodes 16, l7, l8 and 19. With alternating current voltage continuously applied to all of the electrodes extending in the X and Y directions the entire electroluminescent array may be in one operational condition, i.e. the off condition so long as the voltage amplitude of the applied alternating current voltage remains below the threshold voltage value of the threshold switching devices used.

The power source 21 is connected to a pair of start switches 37 and 38 which are engaged together for common actuation, the start switch 37 being connected to a row select switch 39 and the start switch 38 being connected to a column select switch 40 and the start switches 37 and 38 being connected to resistors 22 and 24 respectively to provide an increased potential of alternating current voltage at a selected one of the X lines 11-14 and to a selected one of the Y lines 16-19. The amplitude of the voltage across the resistors 22, 23, and 24 is greater than that of the amplitude across resistor 22 and it is this increased voltage which is applied to selected lines of the electroluminescent panel for a short period of time to energize the selected electroluminescent circuit on the panel. The row select switch 39 has a plurality of contacts which are connected to X lines 11, 12, 13 and 14 via circuit lines 41, 42, 43 and 44 respectively and the column select switch 411 has a plurality of contacts which are connected to Y lines 16, 17, 1% and 19 via circuit lines 46, 47, 48 and 49 respectively, the circuit arrangement of FIG. 1 being only one simplified means for manually operating an electroluminescent array in accordance with this invention.

To energize a selected electroluminescent circuit 20 at a desired juncture of the electroluminescent array 10 the row select switch 39 and column select switch 40 are positioned so as to engage the desired lines. For example, as seen in FIG. 1, row select switch 39 is shown connected to the X electrode 11 while column switch 10 is shown connected to the Y electrode 16. Therefore, the electroluminescent circuit 20 at the juncture 11-16 will be energized when the start switch 37-38 is actuated to apply the full potential of the power source 21 across the electrodes. Although it is obvious to assume that more than one cycle of the full alternating current voltage may be applied to the selected electroluminescent circuit this is merely because of the slow operation of the manually operated start witch. All that is required to render the electroluminescent circuit operative at any particular juncture is one pulse, a positive half cycle or a negative half cycle of sufficient amplitude and thereafter the selected electroluminescent circuit will continue to be energized by the continuously applied reduced voltage from resistor 23.

A stop switch 50 is connected in series with a resistor 51 which together therewith provide a shunt path for the electroluminescent circuit at the juncture selected by row select switch 39 and column select switch 40 and when the stop switch 511 is closed it will reduce the voltage amplitude across the selected lines and discharge stored energy within the electroluminescent circuit to a sufficiently low value so that the electroluminescent circuit will no longer be energized. The term energize or energization of electroluminescent circuits is meant to refer to the second state or ON condition of the electroluminescent circuit during which light is emitted. Although the electroluminescent circuit is pulse generated by the successive half-cycles of the applied alternating current voltage, the electroluminescent circuit will appear to emit light continuously during the energization thereof.

For a better understanding of the operation of the circuit arrangement of FIG. 1 reference is now made to FIG. 3 which illustrates the several circuit components used to form the electroluminescent circuit 20 connected at juncture 11-16, it being understood that all of the electroluminescent circuits 20 are constructed in the same manner. The electroluminescent circuit 20 includes an electroluminescent element 611 for emitting light therefrom, a bi-directional threshold switching device 61 which has inherent time delay for controlling current flow through the electroluminescent element 60, and a resistor 62 to provide suitable current limiting for the circuit and to form the desired RC time constant. However, it will be understood that the resistor 62 may be a discrete component element or may be formed by inherent resistance in the circuit.

With reference to FIG. 3, continuous alternating current voltage of an amplitude below the threshold voltage value of the switching device 61 is applied between the X line 11 and the Y line 16. However, the electroluminescent circuit 20 is not energized until the applied alternating current voltage is of an amplitude greater than the normal threshold voltage value of the switching device 61, as for example, when the start switches 37 and 38, of FIG. 1, are actuated to apply at least one half cycle of alternating current voltage of sufficient amplitude across lines 11 and 16 to render the switching device 61 conductive thereby applying a first pulse of charging current to the electroluminescent element 60 which because it is a capacitive element will store the electrical energy and charge to the peak value of the applied voltage. Therefore, when the line 11 is positive with respect to line 16 the charge on the electroluminescent element 60 is positive on the plate 60a adjacent to the switching device 61. During the next half cycle of alternating current voltage the electrode 11 becomes negative and electrode 16 becomes positive. The negative potential applied to electrode 1 1 is added to the positive charge on the electroluminescent element 60 and the combined voltages are equal to or greater than the threshold voltage value of the switching device 61 to again render the switching device conductive and discharge the positive potential from the electroluminescent element 60 and recharge the electroluminescent element with a negative pulse. When the next half cycle of alternating current voltage is applied across the lines 11 and 16, the line 11 becomes positive and this positive potential is added with the negative charge on the electroluminescent element 60 to once again render the switching device 61 conductive. This action will repeat every half cycle of the applied alternating current voltage only after a pulse of sufficient amplitude has been applied across the juncture to initially render the switching device 61 conductive and provide a charge of electrical energy on the electroluminescent element 60, this being true regardless of the frequency of the applied alternating current voltage.

However, in accordance with this invention the threshold switching device 61 has an inherent time delay between the time a threshold voltage is applied thereto and the time the switching device actually is rendered conductive, and this time delay being inversely proportional to the amount of over-voltage applied to the threshold switching device, as illustrated in FIG. 3A by the curve 63. Here the normal threshold voltage value of a typical threshold voltage switching device is indicated at V and the normal time delay at voltage V id indicated at T By way of example, the normal time delay may be about seconds and the normal threshold voltage value may be about 21 volts, these values being alterable by changing the compositions of the semiconductor material used in forming the switching devices or by varying the thickness of the layers or films formed of such semiconductor materials. However, it will be noted that the time delay T will decrease with increases of applied voltage between V, and V Therefore, the time duration of the start pulse applied to any one of the electroluminescent circuits 20 via the switches 37 and 38 need be only as long as the time delay corresponding to the time delay for the voltage value in excess of V FIG. 38 illustrates the inherent recovery time delay to the normal threshold voltage value of the threshold switching devices after such devices are rendered nonconductive, this being indicated by the curve 64. Here it can be seen that immediately after a threshold switching device is rendered non-conductive it will have a substantially reduced temporary threshold voltage value which increases with time until the normal threshold voltage value is again reached, this being somewhere in the order of 8 to l5 microseconds depending on, among other things, the composition of the materials used to form the semiconductor materials, it being understood that lesser or greater recovery time delays may be involved. Therefore, if a subsequent pulse of voltage is applied to the electroluminescent circuit 20 of FIG. 3 before the threshold switching device 61 has fully recovered to its normal threshold voltage value, this subsequent pulse of voltage need only have an amplitude equal to the then existing threshold voltage value, which may be anywhere between 0.1 V to V depending upon the point in time the next pulse of voltage is applied. The turn-0n time delay of FIG. 3A will persist regardless of the time at which the subsequent pulse of voltage is applied the only difference being a decrease or shifting of the entire curve 63 as indicated by the broken line curves 63a. Therefore, in accordance with this invention, once the threshold switching device 61 is rendered conductive the combined voltages of the applied voltage and the voltage stored on the electroluminescent element 60 can be less than the normal threshold voltage value V of the switching device 61 when the applied alternating current voltage has a frequency such that each half cycle occurs within the time interval T of FIG. 3A.

Also in accordance with this invention the amplitude of the continuously applied alternating current voltage may be selected so that the maximum value of each half cycle of the applied voltage will persist for a period of time sufficient to allow all of the threshold switching devices within the electroluminescent array 10, when selected, to be rendered conductive regardless of their then existing threshold voltage value, thus applying substantially the same voltage to each and every one of the electroluminescent elements 60, the advantageous result being uniform light output from each element. This is best illustrated in FIG. 3C wherein one pulse of voltage of a continuously applied alternating current voltage is indicated by reference numeral 63b and the heavy line top portion 630 being the maximum voltage portion during which all the threshold switching devices will be rendered conductive, this taking place between time points t, and t, as indicated in FIG. 3C. During the period of time between t, and t, there exists only a relatively small change in voltage which is indicated by AV which will be the only voltage variation sensed by the electroluminescent element 60.

Although the description of the electroluminescent circuit has, thus far, been in connection with an alternating current voltage of the sine wave type it will be understood that the continuously applied operating voltage may be of any desired wave shape, as for example, square wave of sawtooth-wave or the like, and the applied voltage may be a varying direct current voltage of a single polarity or an alternating current voltage of alternatively opposite polarities.

Once energized the electroluminescent circuit 26 will continue to be energized by each successive pulse of voltage to emit light either until power is removed from across the lines 11 and 16 or until switch 511, of FIG. 1, is closed to cause a suitable stop pulse to be applied across lines 11 and 16 to discharge the voltage stored within the electroluminescent element 611, this action allowing the threshold switching device 61 to recover fully to its normal threshold voltage value. Once the electroluminescent element, or capacitor, is discharged the next half cycle of alternating current voltage applied across lines 11 and 16 will not render the switching device 61 conductive to energize the electroluminescent circuit 26. Accordingly, it has been shown that the electroluminescent circuit 211 has two stable states of operation when connected to a source of continuously applied alternating current voltage of an amplitude below the threshold voltage value of the switching device 61, and which may even be less than one half of the normal threshold voltage value of the switching device 61 when a preselected frequency to operate the array 10 provides pulses at intervals within the inherent recovery time delay of the switching devices involved.

Seen in FIG. 2 is a single block diagram of an automatic energization and start-stop control circuit which is indicated generally by reference numeral 65 and which is used to control the electroluminescent array 10 in accordance with the principles of this invention. The control circuit 65 provides means for continuously applying alternating current voltage between the X lines 11-14 and the Y lines 16-11 in one form of the invention the continuously applied alternating current voltage may have a sawtooth waveform as indicated in FIG. 4 by reference numeral 66. Additionally, the control circuit 65 includes means for selectively applying start and stop pulses to desired ones of the lines 11-14 and 16-19.

Referring now to FIG. 4, which shows a nonsinusoidal kind of waveform as a continuous energizing voltage which can be used in accordance with one method contemplated by this invention, there is seen sawtooth waveforms 66 which have peak amplitudes of A: V and l6 V where V is the normal threshold voltage value of the switching device 61. A start pulse 67 is generated by the control circuit 6.6 to coincide with the peak amplitude of a positive half cycle 66a of the waveforms 66 and to have an amplitude and time duration sufficient to render conductive the switching device 61 of a selected electroluminescent circuit 21). The amplitude of the start pulse may be +3/2 V and a time duration of about 10' seconds which is sufficient to render the switching device 61 conductive regardless of the polarity of the start pulse with respect to the polarity of the particular coincident half cycle of the waveform 66. Therefore, if the start pulse 67 is coincident with a negative half cycle of the waveform 66 the sum +3/2 V of the start pulse and V of the waveform 66 will equal V whereat the maximum time delay of 10" seconds is required, and the threshold switching device 61 will be rendered conductive. The start pulse 67 may be of negative polarity and will effect operation of the switching device 61 in the same manner.

During the time period t t of FIG. 4, before the start pulse 67 is applied to the waveform 66, the voltage waveform across the switching device 61 is similar to the input waveform 66 as indicated by reference numeral 68. At time t, the electroluminescent circuit 20 is energized and during the time period t,t the waveform across the switching device 61 is indicated by reference numeral 69 illustrating the sloping voltage rise 69a and 6% which terminates at a voltage value of either +V or V and sharply fall along the broken lines 69c and 69d respectively which indicates the rapid switching condition of the switching device 61. The start pulse 67a which appears across the switching device 61 is shown having a peak value of V while the start pulse 67 on waveform 66 has a peak value of 3/2 V This is because once the threshold voltage value is reached the voltage value above the threshold voltage value is rapidly reduced due to the high current flow through the switching device 61. After the start pulse 67 has rendered the switching device 61 conductive for the first time the input waveform 66 is substantially the same during time period t t as it was during the time period t t, but during the time period t -t the electroluminescent element is energized thus being in the stable ON operating condition.

The start pulse 67 also causes a voltage spike 67b to appear across the electroluminescent element and a current pulse 67c to pass through the electroluminescent element. The voltage spike 67b has a voltage value of +3/2 V and is of short time duration as compared to the applied sawtooth waveform 66 and when the start pulse 67 is terminated, since the threshold voltage value of the switching device 61 is V the switching device 61 will again fire in the opposite current conducting direction to reduce the voltage across the electroluminescent element 61) from 3/2 V to V After the electroluminescent element 60 has discharged from +3/2 V to 16 V the charge on the electroluminescent element remains substantially constant as indicated by the positive square pulse a it being understood that minor leakage currents may be involved to slightly reduce the change on the electroluminescent element. When the following half cycle 66b of the waveform 66 is applied across the switching device 61 it is added with the positive charge on the electroluminescent element 60- such that the sum of these two voltages is sufficient to render the switching device 61 conductive to discharge the positive voltage on the electroluminescent element and to recharge the electroluminescent element with a negative voltage equal in amplitude to the negative half cycle 66b, as indicated by the negative square pulse 70b of waveforms 76, and this action will generate a current pulse 72 through the electroluminescent element 60. The dotted vertical lines of waveforms 69 and 70 indicate the rapid switching time of the switching device 61. The electroluminescent circuit 20 will continue to operate in the manner described hereinabove for each half cycle of the applied alternating current voltage to cause alternate polarity current pulses 73, 74, 75, 76 and 77 to pass through the electroluminescent element 60.

At time 2 a stop pulse 80 is generated by the control circuit 65 to coincide with the zero crossing of the waveform 66. The stop pulse 80 is of opposite polarity than the preceding half cycle of the applied alternating current voltage. This is illustrated by the positive half cycle of the waveform 66 preceding the negative stop pulse 80. The stop pulse 80 is of the desired time duration and may have an amplitude of 3/2 V and when added to the 1% V charge on the electroluminescent element 60 the total voltage across the switching device 61 is 2 V and the switching device 61 conducts as indicated by the negative spike 80a on the waveform 69. This will cause the electroluminescent element 60 to discharge its positive voltage and to recharge to a negative voltage value of 3/2 V which causes a negative current pulse 81 to pass through the electroluminescent element 60. Since the stop pulse 80 occurs at the zero crossing of the applied alternating current voltage, (which, therefore, must be at or near zero for at least the turnon delay period of the switching device 61), the 3/ 2 V charge on the electroluminescent element 60 will again render the switching device 61 conductive to cause a second current pulse 82 of opposite polarity which substantially immediately follows the current pulse 81 thereby completely discharging the stored energy from the electroluminescent element 60 and return the electroluminescent circuit 20 to its deenergized stable OFF condition while the alternating current voltage is still applied thereto.

In the form of the invention illustrated by FIGS. 2 and 4 the start and stop pulses developed by the control circuit 65 are of sufficiently long time duration to render the switching device 61 conductive but also of sufficiently short time duration so that the voltage value of the applied alternating current voltage 66 immediately before and immediately after the start and stop pulses are substantially the same.

If the frequency of the sawtooth waveform 66 is selected to operate each of the threshold switching devices 61 prior to their normal inherent recovery time delay td the peak-to-peak value of the continuously applied voltage may be reduced to the value of the then existing threshold voltage value of the switch devices 61 when operated at the selected frequency. Also, the half cycle of voltage immediately following the stop pulse 80 may be required to be eliminated or otherwise compensated for to insure ample time for the threshold switching devices 61 to recover to a threshold voltage value greater than the applied voltage. That is, the threshold switching device may be required fully to recover to the normal threshold voltage value.

Referring now to FIG. 5 there is shown a control circuit indicated generally by reference numeral 85 which can be used to control the energization of discrete points on the electroluminescent array in accordance with this invention. Here the control circuit 85 includes a power drive circuit 86 which generates first and second alternating current voltages such that the voltages are 180 degrees out of phase with respect to each other. The liens 11, 12, 13 and 14 are connected to the power drive circuit 86 through secondary windings 87, 88, 89, and respectively, of pulse coupling transformers 91, 92, 93 and 94, and the lines 16, 17, 18 and 19 are connected to the power drive circuit 86 through secondary winding 96, 97, 98 and 99 respectively of pulse coupling transformers 100, 101, 102 and 103.

A line-select control circuit 104 is provided for applying start and stop pulses to selected ones of the row lines 11, 12, 13 and 14 and includes a start-stop gate 106 and a selector switch circuit 107. The selector switch circuit 107 is connected to primary windings 108, 109, 110 and 111 of transformers 91, 92, 93 and 94 respectively. Synchronizing pulses from the power drive circuit 86 are delivered to the start-stop gate 106 via a line 112 and from the start-stop gate 106 to the selector switch circuit 107 via a line 113.

Also provided is a line-select control circuit 116 which includes a start-stop gate 117 and a selector switch circuit 118 which, in turn, is connected to primary windings 119, 120, 121 and 122 of the transformers 100, 101, 102 and 103 respectively. Synchronizing pulses from the power driving circuit 86 are delivered to the start-stop gate 117 via a line 123, and start and stop pulses are applied to the selector switch circuit 118 via a line 124. The start and stop pulses from the start-stop gates 106 and 117 are developed such that the start pulses, when applied to a corresponding row line and a corresponding column line through their respective transformers, are substantially coincidence with the peak value of the alternating current voltage from the power drive circuit 86, and the stop pulses are substantially coincident with the zero crossing of the applied alternating current voltage. Furthermore, in this embodiment and in accordance with the method of this invention the start and stop pulses applied to the row circuit lines 11-14 are of opposite polarity with respect to the start and stop pulses applied to the column circuit line 16-19 in the same manner as is the applied alternating current voltage connected to these lines.

To provide simultaneous start or stop pulses at selected junctures of the electroluminescent array 10, the start-stop gates 106 and 117 are ganged together by switch means as indicated by the broken line designated by reference numeral 126. A clock pulse generator 127 provides fixed frequency clock pulses which are used to trigger one or more FLIP-FLOP circuits within the power drive circuit 86 to develop the first and second square wave alternating current voltages for operating the electroluminescent array 10.

For a better understanding of the circuit arrangement of the power drive circuit 86 reference is now made to FIG. 6 which shows portions of the power drive circuit 86 in block diagram form and other portions thereof in schematic form thus illustrating one exemplary form of power drive circuit. Reference will also be made to FIG. 7 which illustrates the several different waveforms that are generated at various circuit points and by different circuit components within the power drive circuit 86. Clock pulses 130 from the clock generator 127 are delivered to a Flip-Flop circuit131 to develop at an output 132 of the Flip-Flop 131 square wave pulses 133 and each pulse thereof is designated by reference latter A. The square wave output 133 of the Flip-Flop 131 is applied to the input of a Flip-Flop 124 and to a pair of AND gates 136 and 137. Each pulse A of the square wave pulses 133 will cause a change of state within the Flip-Flop 134 to develop at the output thereof square wave pulses 138 and 139 which are applied to another pair of inputs of the AND gates 136 and 137 respectively. Each pulse of the square wave pulses 138 is designated by reference letter B while each pulse of the square wave pulses 139 is designated by the reference letter D, and the output of the AND gates 136 and 137 will be the time point summation of the A, B and 13 pulses appearing at the input, as is well known in the art. Therefore, at-the output of the AND gate 136 there will be developed square wave pulses 140 each pulse of which is designated A+B signifying the addition of the A and B pulses, and at the output of the AND gate 137 there will appear square wave pulses 141 and each pulse thereof is appropriately designated A-l-fi but displaced in time with respect to pulses A+B.

An inverter circuit 142 has a first transistor 143 for receiving the A+B pulses at the base electrode thereof and, in the same manner, a second transistor 144 receives the A+l 3 pulses so that the transistors 143 and 144 are alternately rendered conductive to cause current to flow through one-half of primary winding 146 of an output transformer 147 during one time interval and to cause current to flow through the other half of the primary winding 146 during a subsequent time interval. The center tap of the primary winding 146 is connected to the positive terminal of a power source, not shown, while the emitter electrodes of transistors 143 and 144 are connected to the negative terminal of the power source. A secondary winding 148 of the transformer 147 has a center tap 151 connected to ground potential and a pair of opposite end leads 149 and 150 connected to the row or X lines and to the column or Y lines respectively through their respective transformer secondary windings 87-90 and 9699 as seen in FIG. 5. The nature of the alternating current voltage developed at lines 149 and 150 is best illustrated by the waveforms 152 and 153 which show the output of line 149 and the output of line 150 as being 180 degrees out of phase 'with respect to one another.

The start and stop pulses originate within the power drive circuit 86 at the output of the AND gates 136 and 137 thereby being synchronized with corresponding half cycles of the alternating current voltages represented by wave shapes 152 and 153. The outputs of the AND gates 136 and 137 are connected to the start-stop gates 106 and 117 by way of a pair of lines 156 and 157, respectively, and it is the start-stop gate circuits 106 and 117 which determine the characteristic of the pulse, whether it be a start pulse or a stop pulse.

FIG. 8 illustrates in schematic form one kind of circuit arrangement which can be used as the start-stop gates 106 and 117 and the selector switch circuits 107 and 118 to apply pulses of the proper polarity and at the appropriate time for controlling operation of the electroluminescent array 10. The A+B pulses of FIG. 6 are applied to the base electrodes of transistors 160 and 161 which are of PNP and NPN types respectively. The collector electrode of transistor 160 is connected to ground potential through a resistor 162 and the emitter electrode of transistor 160 is connected to a positive voltage source and, therefore, a position A+B pulse at the base electrode transistor 160 will cause a positive pulse to pass through a capacitor 163 and be applied to the selector switch circuit 118. On the other hand, the collector electrode transistor 161 is connected to a positive voltage source through a resistor 164 and the emitter electrode of transistor 161 is connected to ground potential and, therefore, the positive A+B at the base electrode of transistor 161 will cause a negative pulse to pass through a capacitor 166 which is then applied to the selector switch circuit 107. The positive pulse from capacitor 163 is applied to the selector switch circuit 118 through a start-stop switch 167, and the negative pulse from capacitor 166 is applied to the selector switch circuit 107 through a start-stop switch 168 which is ganged together with switch 167 as indicated by the broken line 126. When switches 167 and 168 are in the position shown in FIG. 8, the positive and negative pulses applied to the selector switch circuits 118 and 107 respectively will be coincident with the peak value of half cycles of one polarity of the alternating current voltage applied to the X and Y lines to energize desired electroluminescent circuits 20. However, if the pulses from the capacitors 163 and 166 are to be stop pulses, the switches 167 and 168 are moved to the stop position thereby causing the positive pulse at switch 167 to pass through a pulser and delay circuit 169 and the negative pulse at switch 168 to pass through a pulser and delay circuit 170. The pulser and delay circuits 169 and 170 transform the stop pulses from the capacitor 163 and 166 to change the polarity of the pulses and to cause the pulses, as illustrated in FIG. 9, to be coincident with the beginning of 5 new half cycle wherein the alternating current voltage applied to the electroluminescent array 10 is zero and is of a sufficient time duration to discharge the selected electroluminescent elements on the array 10 thus completely de-energize the electroluminescent circuit The A+ pulses from FIG. 6 are applied to the base electrodes of transistors 171 and 172 which are NPN and PNP types respectively. The collector electrode of transistor 171 is connected to a positive potential through a resistor 173 and the emitter electrode of transistor 171 is connected to ground potential and, therefore, a positive A+I 3 pulse at the base of the electrode thereof will cause a negative pulse to pass through a capacitor 174 and be applied to the selector switch 118. The collector electrode of transistor 172 is connected to ground potential through a resistor 176 and the emitter electrode of transistor 172 is connected to a positive voltage source and, therefore, the positive A+B pulse at the base electrode of the transistor 172 will cause a positive pulse to pass through a capacitor 177. The positive and negative pulses from capacitors 177 and 174 respectively are treated in the same manner as the positive and negative pulses from capacitors 163 and 166 the only difference being that the pulses from capacitors 177 and 174 occur during half cycles of different polarity than the half cycles during which pulses from capacitors 163 and 166 occur. Therefore, start and stop pulses are generated during each half cycle of the applied alternating current voltage thereby providing control for the electroluminescent circuits 20 at time intervals as short as the time intervals between half cycles of the applied alternating current voltage. This provides for rapid changing of display patterns which are produced on the electroluminescent array 10.

Pulses which are applied to the selector switch circuit 118 are selectively delivered to primary windings 119-122 by actuation of switches 180, 181, 182 and 183 any one or all of which may be actuated in any manner desired, as for example, manually or electrically. Similarly, start and stop pulses which are applied to the selector switch circuit 107 are selectively delivered to the primary windings 108-111 by actuation of switches 186, 187, 188 and 189 any one or all of which may be actuated in a manner as mentioned hereinabove. For example, if the electroluminescent circuit 20 at juncture ill-16 is to be energized, switches 180 and 186 are actuated while switches 167 and 168 are in the start position. On the other hand, if the electroluminescent circuit 20 at the juncture till-l6 is to be de-energized, switches 180 and 186 are actuated while the start-stop switches 167 and 168 are in the stop position. The waveforms 152 and 153 of P16. 6 have peak voltage amplitudes which are less than one half of the threshold voltage value of the switching device 61 shown in FIG. 3 and when, for example, the positive half cycle of the waveform 152 is applied to the X lines of the array and the corresponding negative half cycle of waveform 153 is applied to the'Y lines of the array 10 the total voltage impressed across each of the threshold voltage value greater than the voltage applied across the electroluminescent circuits 20. By actuation of a selected one of the switches 180-183 and a selected one of the switches 1867189 a start pulse will be applied, for example, to a positive half cycle of the waveforms 152 and to a negative half cycle of the waveforms 153 as indicated by reference numerals H90 and 191 shown in FIG. 9. The sum of the peak amplitude of the start pulses 190 or 191 and the associated waveforms 152 or 153 is greater than one half but less than the threshold voltage value of the threshold switching device associated with the electroluminescent circuits so that when the start pulses 190 or 191 are added together with such waveform the combined voltage across the selected juncture will exceed the threshold voltage value of the switch at that juncture to initiate energization of the particular electroluminescent circuit but no other juncture will sense a voltage greater than the threshold voltage value of the switching devices. This action will cause a resultant voltage charge equal in amplitude to the combined voltages of the start pulses 1190 or 191 and waveforms 152 or 153 to be stored in the electroluminescent element as indicated by a voltage pulse 192 of waveforms 193. Also, when the threshold switching device of the selected electroluminescent circuit 20 is rendered conductive a current pulse 19d passes through the associated electroluminescent element and causes the element to emit light. The next half cycles 152a and 153a of the continuously applied alternating current waveforms 152 and 153 will add with the voltage charge on the electroluminescent element to again render the threshold switching device conductive and discharge the positive voltage pulse 192 and recharge the electroluminescent element with a negative voltage pulse 196. This will cause a corresponding negative current pulse 197 to pass through the electroluminescent element. With no further application of start pulses the selective electroluminescent circuits 20 will remain energized while non-selected electroluminescent circuits 20 will remain de-energized. The reason selected electroluminescent circuits 20 remain energized is evident from fact that the voltage charge on the electroluminescent element is greater than one half but less than the threshold voltage value of the threshold switching device and the applied alternating current voltage is greater than one half but less than the threshold voltage value of the threshold switching device so that during energization of the electroluminescent circuit 20 the combined voltage amplitudes across the threshold switching device during each half cycle is greater than the threshold voltage value of the switching device. However, if the frequency of the applied alternating current voltage is selected so that each half cycle of the applied voltage occurs within the time period of the recovery time delay of the threshold switching devices the combined amplitude of the voltage on the electroluminescent element 60 and the applied voltage need be only greater than the then existing threshold voltage value of the threshold switching devices associated with the energized electroluthan the preceding pulses 203 and 204 respective'lyQ However, the stop pulses 200 and 201 are of sufficiently short time duration so that the positive voltage pulse 206 on the electroluminescent element is discharged and no further charge will be applied to the electroluminescent element thereby de-energizing the electroluminescent circuit 20, but the time duration of the stop pulse 200 or 201 or at least the off time of the threshold switching devices involved must be sufficiently long to insure recovery of thethreshold voltage value to a level greater than the applied voltage. During final discharge of the selected electroluminescent element the final voltage pulse 206 will be of shorter time duration. Also, the period during which the waveforms are at or near zero is at least equal to the turn-on delay period of the threshold switching devices so that the stop pulse 200 or 201 can be effective on the switch device during these zero periods.

Although the switches- 167, 168, -183 and.

186-189 are shown as manually operated switches it will be understood that the actual use of these switches will be electronic switches to provide rapid switching time and rapid selection of the desired switches. 

1. A circuit arrangement for controlling energization of discrete elemental points on an electroluminescent array comprising: a plurality of row circuit lines and a plurality of column circuit lines, each of said lines arranged with respect to the other to form circuit junctures corresponding in number to the number of the discrete elemental points on the electroluminescent array; circuits having electroluminescent element energy storage means physically located at a different discrete points on said array and threshold switching means at said points operable at a predetermined voltage amplitude after an inherent turn-on time delay; means for developing first and second alternating current voltages, said second alternating current voltage being 180 degrees out of phase with respect to said first alternating current voltage, said first and second alternating current voltages having an amplitude less than one half said predetermined voltage amplitude; means for applying said first alternating current voltage to said plurality of row circuit lines; means for applying said second alternating current voltage to said plurality of column circuit lines; and address circuit means selectively electrically connectable to selected ones of said row circuit lines and selected ones of said column circuit lines, said address circuit means hAving first means for generating start pulse signals of substantially the same amplitude for producing across a selected row and column circuit line a resultant signal of an amplitude greater than said predetermined voltage amplitude, and of a time duration at least equal to the inherent turn-on time delay of said threshold switching means, and having second means for generating stop pulse signals of substantially the same amplitude for rendering said threshold switching means non-conductive for a period of time sufficient to enable said threshold switching means to recover to a threshold voltage value greater than the voltage value of the applied alternating current voltage, and including select switch means to apply said start pulses to a selected row circuit line and to a selected column circuit line during one instant to render conductive the threshold switching means at the selected juncture to charge said electroluminescent element energy storage means to initiate energization of the selected electroluminescent circuit, and to apply said stop pulses to a said selected row circuit lines and a selected column circuit line during another instance to render the threshold switching means conductive for a short period of time sufficient to discharge said energy storage means and thereafter maintain said threshold switching means non-conductive until it recovers to its original threshold voltage value to de-energize said electroluminescent circuit at said juncture, said initially energized electroluminescent circuit maintaining its energized state due to the repeated summation of the stored energy in said electroluminescent element energy storage means and the applied alternating circuit voltage.
 2. The circuit arrangement for controlling energization of discrete elemental points on an electroluminescent array according to claim 1 wherein said first and second alternating current voltages are square in wave shape.
 3. The circuit arrangement for controlling energization of discrete elemental points on an electroluminescent array according to claim 1 wherein said stop pulse signals are coincident with the time said applied alternating current voltage is at or near zero and of the opposite polarity with respect to a preceding half cycle.
 4. The circuit arrangement for controlling energization of discrete elemental points on an electroluminescent array according to claim 1 wherein said start pulse signals are coincident with the time said applied alternating current voltage is at or near zero and of the same polarity as the preceding half cycle.
 5. A method of controlling energization of discrete points on a light-emitting array wherein there is formed a plurality of pairs of circuit junctures, a light-emitting circuit across each of said pairs of circuit junctures, each of said light-emitting circuits including light-emitting means distributed over a display area and connected in circuit with a voltage amplitude and time delay responsive threshold switch means initially rendered conductive when a voltage of at least a given initial threshold voltage value is applied thereto for a given minimum period and reverting to a non-conductive state when the applied current therethrough drops to a value below a given minimum holding current value, said threshold switch means having a recovery time delay characteristic wherein the operating voltage amplitude necessary to render the same conductive immediately after it is rendered momentarily conductive and then becomes non-conductive suddenly drops to a minimum temporary threshold voltage value and then gradually rises to said given initial threshold voltage value over a given recovery delay period, the method comprising the steps of: providing a repetitively pulsating voltage waveform across said circuit junctures of an amplitude below said given initial threshold voltage value but of an amplitude and frequency to provide for each of said threshold switch means, over a given base period thereof after it is initially rendered cOnductive and then non-conductive, voltage pulsations spaced apart less than the recovery delay period of said threshold switch means and having an amplitude at or greater than the then existing time increasing temporary threshold voltage value thereof, momentarily to render the threshold switch means again conductive; and applying a resultant start pulse across a selected threshold switching means a voltage at least equal to said given initial threshold voltage value and for a period to render said threshold switch means momentarily conductive, thereby thereafter to effect the repeated feeding of energizing current to the light-emitting circuit at said selected pair of circuit junctures by means of said pulsating voltage while light-emitting circuits at non-selected junctures remain de-energized.
 6. In combination: a voltage amplitude and time delay responsive threshold switch means initially rendered conductive when a voltage of at least a given initial threshold voltage value is applied thereto for a given minimum period and reverting to a non-conductive state when the applied current therethrough drops to a value below a given minimum holding current value; said threshold switch means having a recovery time delay characteristic wherein the operating voltage amplitude necessary to render the same conductive immediately after it is rendered momentarily conductive and then becomes non-conductive suddenly drops to a minimum abnormal temporary threshold voltage value and then gradually rises to said given initial threshold voltage value over a given recovery delay period; a source of a continuous voltage having an amplitude below said initial threshold voltage value and which effects the presence across said threshold switch means of a repetitively pulsating voltage where the voltage pulsations have an amplitude below said initial threshold voltage value and are spaced apart intervals less than the recovery delay period of said threshold switch means so the voltage pulsations will continue repeatedly to drive said threshold switch means momentarily into conduction after the threshold switch means has been initially triggered into a momentary conductive state; start means for initially momentarily raising the voltage applied to said threshold switch means to said initial threshold voltage value momentarily initially to render the same conductive; and stop means for selectively keeping the threshold switch means non-conductive for a period to permit said increasing threshold voltage value to rise above the amplitude of the voltage resulting from said source of a continuous voltage.
 7. The combination of claim 6 wherein there is capacitive means in series with said threshold switch means which capacitive means charges up to the peak value of said source of continuous voltage to render the threshold switch means non-conductive, said source of continuous voltage being a source of alternating current where successive pulsations thereof are of opposite polarity, each of which pulsations adds to the voltage built up on the capacitive means during a preceding period.
 8. The combination of claim 6 wherein there is provided a plurality of row circuit lines and a plurality of column circuit lines; said source of continuous energizing voltage being continuously applied across the different combinations of row and column circuit lines, there being a multiplicity of said threshold switch means, one such threshold switch means being respectively connected in series with each selected row circuit and column circuit line to provide a storage array, said starting means including means for respectively producing a pair of start pulses of opposite phase, and means for selectively coupling said pair of start pulses of opposite phase respectively in series with the selected row and column circuit line where the voltage pulses are in additive relationship between the selected row and column circuit line.
 9. The information storage array of claim 8 wherein there is provided capacitive meAns in series with each threshold switching means which capacitive means becomes repeatedly charged to the peak value of the applied voltage thereby to reduce the current flow through the threshold switching means to zero each time the threshold switch means is rendered conductive, the voltage charge on the capacitive means adding to the next opposite polarity voltage pulsation of the alternating voltage to provide a resultant voltage which exceeds the threshold voltage value of the associated threshold switching means.
 10. An information storage array comprising: a plurality of row circuit lines and a plurality of column circuit lines; a separate bistable circuit connected between each row and each column circuit line to form an array; each bistable circuit including bidirectional threshold switching means normally blocking the flow of current until a voltage is applied thereto which exceeds a threshold voltage value for a given turn-on delay period which operates the same into a conductive state, the threshold switch means becoming non-conductive when current flow therein drops to a value below a given minimum holding current value, alternating voltage pulsation supplying means for providing a continuous alternating voltage waveform across said row and column circuit lines across which said bistable circuits are connected and which voltage waveform varies in polarity and at least during certain desired instants has a portion of its waveform at or near zero for a finite period equal at least to said given turn-on delay period, the magnitude of said alternating voltage waveform being of a value less than said threshold voltage value; momentarily operating bistable circuit starting means for selectively applying a pair of start voltage pulses of opposite phase respectively to a selected row and column circuit line which produces a resultant voltage exceeding said threshold voltage value of the threshold switching means of the bistable circuit connected between the selected row and column circuit line which threshold switching means becomes momentarily conductive to initiate an ON bistable mode of operation where the threshold switching means is rendered momentarily repeatedly conductive by the voltage pulsations involved supplied by said alternating voltage source means until the bistable circuit is operated to a stable OFF mode of operation by application of a resultant turn-off pulse thereto; and momentarily operating bistable circuit stopping means for momentarily applying a resultant turn-off pulse to a selected row and column circuit line which effects said turn-off mode of operation where said alternating voltage waveform is ineffectual in rendering said threshold switching means conductive, the pulses generated by at least one of said bistable circuit starting and stopping means occurring at a time when said alternating voltage waveform is at or near zero.
 11. The information storage array of claim 10 wherein said bistable circuit starting means produces said voltage pulses across each selected pair of row and column circuit lines during the time said alternating voltage waveform is at or near zero.
 12. The information storage array of claim 11 wherein said bistable circuit starting means produces said start pulses across the selected pair of row and column circuit lines at a point in time where it is of the same polarity as the immediately preceding portion of said alternating voltage waveform.
 13. The information storage array of claim 11 wherein said bistable circuit stopping means produces said resultant turn-off pulse across the selected pair of row and column circuit lines at a point in time where it is of the same polarity as the immediately succeeding portion of said alternating voltage waveform.
 14. The information storage array of claim 10 wherein each bistable circuit includes light emitting means which emits light during one of said stable conditions thereof.
 15. The information storage array of claim 10 wherein said alternating voltage supplYing means comprise rectangular pulses alternating in polarity, the rectangular pulses being spaced from one another so the alternating voltage waveform between succeeding pulses remains at zero for at least said turn-on delay period.
 16. The information storage array of claim 10 wherein the alternating voltage produces pulses alternating in polarity during successive half cycles, the threshold switching means of each bistable circuit has a recovery time delay characteristic wherein the threshold voltage value thereof immediately after the conductivity thereof is terminated drops to a minimum temporary threshold voltage value which gradually rises to said given threshold voltage value over a given recovery delay period, the frequency of said alternating voltage being such that each half cycle thereof is less then the given recovery delay period, so that the amplitude of the voltage pulsations of said alternating voltage may initiate conduction of the associated threshold switching means each half cycle at an amplitude below said initial threshold voltage value once the threshold switching means is initially rendered momentarily conductive.
 17. An information storage array comprising: a plurality of row circuit lines and a plurality of column circuit lines; a separate bistable circuit connected between each row and each column circuit line to form an array; each bistable circuit including bidirectional threshold switching means normally blocking the flow or current until a voltage is applied thereto which exceeds a threshold voltage value for a given turn-on delay period which operates the same into a conductive state, the threshold switch means becoming non-conductive when current flow therein drops to a value below a given minimum holding current value, alternating voltage supplying means for providing a continuous alternating voltage waveform across said row and column circuit lines across which said bistable circuits are connected and which voltage waveform alternates in polarity, the magnitude of said alternating voltage waveform being of a value less than said threshold voltage value; momentarily operating bistable circuit starting means for selectively applying a pair of start voltage pulses of opposite phase respectively to a selected row and column circuit line which produces a resultant voltage exceeding said threshold voltage value of the threshold switching means of the bistable circuit connected between the selected row and column circuit line which threshold switching means becomes momentarily conductive to initiate an ON bistable mode of operation where the threshold switching means is rendered repeatedly momentarily conductive by the voltage pulsation supplied by said alternating voltage waveform until the bistable circuit is operated to a stable OFF mode of operation by application of a resultant turn-off pulse thereto; said momentarily operating bistable circuit stopping means for momentarily applying a resultant turn-off pulse across a selected row and column circuit line which voltage effects said turn-off mode of operation where said alternating voltage waveform is ineffectual in rendering said threshold switching means conductive.
 18. A method of storing information in a matrix including a plurality of row circuit lines and a plurality of column circuit lines with a bistable circuit connected between each row and column circuit line to form an array of such bistable circuits; each bistable circuit including bidirectional threshold switching means normally blocking the flow of current between the associated row and column circuit line until a voltage is applied thereto which exceeds an initial threshold voltage value for a given turn-on delay period to operate the same into a conductive state, the threshold switch means becoming non-conductive when current flow therein drops to a value below a given minimum holding current value, the method comprising the steps of: continuously applying to the various row and column circuit lines an alternatiNg voltage waveform comprising pulsations which alternate in polarity, the magnitude of said alternating voltage waveform being of a value less than said threshold voltage value so as to have no effect thereon when the bistable circuit is in its OFF bistable condition and being capable of driving the threshold switching means of each bistable circuit momentarily into a conductive state only when the threshold switching means is initially momentarily operated into a conductive state to trigger the bistable circuit into an ON bistable condition said alternating voltage waveform being at or near zero for a number of said turn-on delay periods; applying respective pairs of bistable turn-on pulses of opposite phase during successive intervals all occurring during a time said alternating voltage waveform is at or near zero for a number of said turn-on delay periods, respectively to different pairs of row and column circuit lines, each pair of turn-on pulses providing a resultant voltage pulsation across the selected row and column circuit line which exceeds said initial threshold voltage value of the associated threshold switching means to operate the bistable circuit involved into its ON bistable condition, thereby to sequentially trigger various selected bistable circuits into their ON bistable conditions while said alternating voltage waveform is at or near zero for a number of said turn-on delay periods; and sequentially applying bistable turn-off pulses respectively to different pairs of row and column circuit lines sequentially to reset various selected bistable circuits in their ON bistable conditions to said OFF bistable conditions.
 19. A method of storing the information in a matrix including a plurality of row circuit lines and a plurality of column circuit lines with a bistable circuit connected between each row and column circuit line to form an array of such bistable circuits; each bistable circuit including bidirectional threshold switching means normally blocking the flow of current between the associated row and column circuit line until a voltage is applied thereto which exceeds an initial threshold voltage value for a given turn-on delay period to operate the same into a conductive state, the threshold switch means becoming non-conductive when current flow therein drops to a value below a given minimum holding current value, the method comprising the steps of: continuously applying to the various row and column circuit lines an alternating voltage waveform comprising pulsations which alternate in polarity, the magnitude of said alternating voltage waveform being of a value less than said threshold voltage value so as to have no effect thereon when the bistable circuit is in its OFF bistable condition and being capable of driving the threshold switching means of each bistable circuit momentarily into a conductive state only when the threshold switching means is initially momentarily operated into a conductive state to trigger the bistable circuit into an ON bistable condition; generating a pair of turn-on pulses of opposite phase which, when respectively fed to a selected row and column circuit line, provides a resultant turn-on voltage pulsation across the bistable circuit connected across the selected row and column circuit line which exceeds the initial threshold voltage value of the associated threshold switching means; directing one of said pair of turn-on pulses to one row or column circuit line and the other of said pulses simultaneously to a number of the other column or row circuit lines to apply said resultant turn-on voltage pulsations to a number of bistable circuits simultaneously; generating a pair of bistable turn-off pulses which, when respectively fed to a selected row and column circuit line generates a resultant turn-off voltage pulsation which resets an ON bistable circuit thereacross to its OFF bistable condition; and directing one of said pair of turn-off pulses to one row or column circuit line and the other of said puLses simultaneously to a number of the other column or row circuit lines to apply said resultant turn-off voltage pulsations to a number of bistable circuits simultaneously to reset all of the same.
 20. The method of claim 19 wherein said alternating voltage waveform has at least during certain desired instants thereof a portion at or near zero for a finite period equal at least to said given turn-on delay period, and said pair of bistable turn-on pulses are fed to selected row and column circuit lines during an instant when said alternating voltage waveform is at or near zero for said finite period.
 21. An information storage array comprising: a plurality of row circuit lines and a plurality of column circuit lines; a separate multi-stable state circuit connected between each row and each column circuit line to form an array; each multi-stable state circuit including bidirectional threshold switching means normally blocking the flow of current until voltage is applied thereto which exceeds a threshold voltage value for a given turn-on delay period which operates the same into a conductive state, the threshold switch means becoming non-conductive when current flow therein drops to a value below a given minimum holding circuit value, voltage pulsation supplying means for providing a continuous pulsating voltage waveform of varying polarity across said row and column circuit lines across which said multi-stable state circuits are connected, and at least during certain desired instants has a portion of its waveform at or near zero for a finite period equal at least to said given turn-on delay period, the magnitude of said pulsating voltage waveform being of a value less than said threshold voltage value; momentarily operating multi-stable state circuit starting means for selectively applying a start signal pulse to a selected multi-stable state circuit which produces a resultant turn-on voltage across the associated threshold switch means exceeding said threshold voltage value thereof which threshold switching means becomes momentarily conductive to initiate an ON mode of operation where the threshold switching means is repeatedly rendered momentarily conductive by the voltage pulsations of said pulsating voltage waveform until the multi-stable state circuit is operated to a stable OFF mode of operation by application of a resultant turn-off signal pulse to the multi-stable state circuit involved; and momentarily operating multi-stable state circuit stopping means for momentarily applying a resultant turn-off signal pulse to a selected multi-stable state circuit which effects said turn-off mode of operation where voltage pulsations are ineffectual in rendering said threshold switching means conductive, the pulses generated by at least one of said multi-stable state circuit starting and stopping means occurring at a time when said pulsating voltage waveform is at or near zero.
 22. The information storage array of claim 21 wherein said multi-stable state circuit stopping means feeds said resultant turn-off signal pulse to a selected multi-stable state circuit during the time said pulsating voltage waveform is at or near zero.
 23. A multi-stable state circuit comprising, in combination: a source of continuously generated voltage pulses of varying polarity, the waveform of pulses produced by said source of voltage pulses providing repeated intervals where the waveform is at or near zero; capacitive means; a voltage amplitude responsive bidirectional threshold switch means which is normally in a non-conductive state and switches to a conductive state when a voltage of at least a given threshold voltage value of any polarity is applied thereacross and reverts to a non-conductive state when the current therethrough drops below a given minimum holding current value, said voltage pulses of varying polarity having amplitudes below said threshold voltage value; means connecting said source of voltage pulses, capacitive means and threshold switch means in mutual seriEs circuit relation; first control means selectively momentarily operative to raise the voltage across said threshold switch means to said threshold voltage value to switch the threshold switch means to a conductive state and effect the charging of said capacitive means, said threshold switch means, after said capacitive means has so charged, becoming non-conductive so the capacitive means holds the charge at a value which produces a voltage which will add to a subsequently generated pulse of said source of voltage pulses of a given polarity opposite to that which previously charged the same to again raise the voltage across said threshold switch means to said threshold voltage value, whereupon the threshold switch means again becomes momentarily conductive to effect the initial discharge and then the reverse charging of said capacitive means to a voltage at or near said pulse of said given polarity to condition the circuit for repeated switching of said threshold switch means to a conductive state by the addition of each new voltage charge on said capacitive means with a pulse from said source of pulses of opposite polarity to that which previously charged the same; and second control means selectively operative when said pulses of varying polarity of said source of pulses are repeatedly switching said threshold switch means to said conductive state by effecting the discharge of said capacitive means to a value which will not sustain the switching of said threshold switch means to a conductive state by said pulses of varying polarity; and at least one of said control means is operative during a period when said waveform of pulses is at or near zero.
 24. The circuit of claim 23 in combination with a plurality of row circuit lines and a plurality of column circuit lines; said source of voltage pulses being continuously applied across the different combinations of row and column circuit lines; there being a multiplicity of said circuits, one such circuit being respectively connected in series with each selected row circuit and column circuit line to provide an array of such circuits; said first and second control means each producing control signals; and there is provided means for selectively coupling said control signals to the circuit connected across any selected row and column circuit line. 